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Mycology Laboratory Equipment and Techniques: Pasteurizing Mushroom Substrates via “Cold Sterilization”

“Cold sterilization” is a perennial hot topic when it comes to sterilizing and pasteurizing mushroom substrates. This chemical-based process (usually using low-magnesium hydrated lime) seems like a “best of both worlds” approach. It is cheap, low-labor, and certainly cost-effective, even at extremely high volumes. No wonder it has many enthusiastic boosters and is standard in many commercial operations and industrial applications.

As an autoclave company that has supplied mycology laboratory equipment and mushroom autoclaves to many research organizations and cultivators, we can’t help but lead with a very important distinction: 

“Cold sterilization” isn’t actual sterilization.

So-called “cold sterilization” is actually a form of pasteurization, not sterilization. Pasteurization only heats the substrate between 150 and 180 degrees Fahrenheit for an hour and a half to two hours; whereas sterilization requires heating to 250 degrees Fahrenheit under pressure. Pasteurization will not sterilize—or remove all contaminants—the way actual sterilization will. It really only weakens microorganisms, killing some pathogenic bacteria and reducing the level of substrate contaminants in the process.

Pasteurization can be sufficient with some hearty mushroom varieties (e.g., most popular edible mushrooms, including all the varieties of Oyster mushrooms) that grow in non-supplemented straw substrates, like oat or wheat straw. These really only need a head-start on the other spores in the substrate.

But “cold sterilization” is a huge mess (or entirely ineffective) with most non-straw or supplemented mushroom substrates. And, even with unsupplemented straw, it still doesn’t remove/kill/deactivate all forms of life present in the substrate.

Why Sterilize Non-Straw or Supplemented Substrates?

Pasteurization may suffice for growing oysters—although to optimize your mushroom yield, you might still consider a sterilization process instead (more on that below). But many other mushroom species—reishi, shiitake, lions mane, and button, to name a few—thrive best in different substrates. Different substrates require different kinds of substrate preparation—including supplements—many of which require steam sterilization, not “cold sterilization.”

For instance, grain is a common, nutrient-rich substrate addition. It can generate high yields because every single kernel can inoculate mushroom mycelium that can spread. But the robust growing environment grain creates is also vulnerable to contamination. Mycologist Paul Stamets points out that each gram of rye berries can carry 300,000 bacteria and 12,000 fungi—and some batches host into the millions! Water added to the mushroom substrate encourages growth—but not just mycelium growth. A bacterium or fungus uses the same food as a mushroom to flourish. Enriched substrate will quickly contaminate with mold before mushrooms can colonize unless it’s first sterilized and its sterility maintained during the inoculation process until the mushroom takes hold. But if you sterilize your mushroom substrate, your crop stands a much better chance.

Even still, sterilizing grain for mushroom cultivation can be a challenge. Each grain kernel is honeycombed with air pockets and cavities that harbor unwanted spores and active colonies and trap cold air that can protect contaminants from sterilization. Depending on the type of grain, additives, and volumes involved, grain spawn regularly takes at least an hour or two to sterilize completely, even in an autoclave. A purpose-built, programmable autoclave lets you fine-tune your sterilization process for improved results.

Sterilizing vs. Pasteurizing Mushroom Substrates

Autoclaving doesn’t just outperform “cold sterilization” in terms of removing contamination. Studies have shown sterilized substrates often outperform pasteurized substrates, in terms of total yield, yield per flush, and mushroom quality.

For example, in a 2012 paper published in International Journal of Agriculture and Biology, researchers Tajudeen Oseni and Sikhumbuzo Dlamini found “[t]here were significant effects of substrate pre-treatment methods on the average yield of oyster mushroom…[H]ighest growth vigour, yield[,] and B.E. [biological efficiency] were obtained in autoclaved sugarcane bagasse [substrate]” when compared to substrate treated using pasteurization methods. They also noted greatly reduced time to colonize the substrate—in some cases by half—and improved mushroom quality (with mushrooms grown in autoclaved substrate having a heartier stipe).

Another study found that “There is a significant difference among yield of different sterilization methods. The results revealed that lab autoclave (1hr) proved one of the best sterilization method. … It was observed that the Pleurotus ostreatus [mushrooms grown in autoclaved substrate] gave the maximum yield in the first flush followed by second, third and fourth flush” when grown in autoclave-sterilized substrate.

impact of sterilizing/pasteurizing mushroom substrates on harvest yield

The impact of sterilization/pasteurization on mushroom yields [source]

A pressure cooker may have gotten you started with cultivation in supplemented mushroom substrates, but quickly become unwieldly, time consuming, and inconsistent as your volume expands—-leading you to consider an autoclave. A standard lab autoclave, not designed with mycology in mind, might be too restrictive for your needs. A Priorclave, however, consistently sterilizes challenging loads like grain and other mushroom substrate.

Of course, every lab (and every mushroom) is different. For more than three decades, Priorclave has been building autoclaves to order and supporting labs in developing the right cycles and procedures. Contact us whenever you wish to discuss how your lab can get more with less wasted water, energy, and time.